OP-CHART; Early Bloomers

Published: April 19, 2012

THE naturalist and philosopher Henry David Thoreau coined a wonderful word for an imagined instrument in his 1854 book, ''Walden'': the ''realometer.'' Thoreau's realometer would allow an inquiring person to measure the reality of his perceptions, to push past the ''mud and slush of opinion, and prejudice, and tradition, and delusion, and appearance ... to a hard bottom.''

Thoreau has provided us with this very tool in his extensive journals. Starting in 1851, he began recording the progress of the seasons in Concord, Mass., by noting the first flowers, leaves and migratory birds of spring. All told, he kept records for more than 300 species.

On May 11, 1853, Thoreau recorded the first open flower of the highbush blueberry. Its distinctive white tubular flowers are easy to observe. In subsequent years he recorded the first blueberry flowers in Concord between May 14 and 19.

If Thoreau went looking for the first blueberry flowers of Concord in mid-May today, he would be too late -- some bushes would be covered with flowers, while others would have only a few stragglers left hanging among the young green fruits. Since the 1850s, the first blueberry flowering has shifted three weeks earlier -- the blossoms now generally open during the last two weeks of April. But this year, after a record warm winter, blueberry bushes began to flower on April 1, six weeks earlier than in Thoreau's time.

The flowering times of other species, like the shadbush and marsh marigold, shifted a similarly extreme amount. More species, like birdfoot violet, rhodora and flowering dogwood, changed by only one or two weeks. Some changed even less or not at all. But the shift toward earlier spring flowering is a widespread pattern.

Warming weather in Concord is most likely the cause. Over the last 160 years, April temperatures at the nearby Blue Hill Meteorological Observatory have warmed by around five degrees, because of a combination of global warming and warming associated with the expansion of paved surfaces and buildings in metropolitan Boston. Plants on average flower two days earlier for each degree increase in Concord -- thus, the town's plants are generally flowering about 10 days earlier than when Thoreau made his observations. With temperatures predicted to rise by four to eight additional degrees this century, plants could flower 8 to 16 days earlier than they do now.

Of course, it's not just Concord. Records from every continent and the oceans in between show changes in the timing of plant and animal behaviors, including flowering, mating, migrating and emerging from hibernation. Some species are changing faster, some slower, but the changes matter. Pollinators may arrive too early for their favorite flowers. Predators may arrive too late for their preferred prey. Species will have to adjust or perish. No doubt, there will be -- and already are -- winners and losers in this great shake-up.

In Concord, we were unable to find many of the wildflowers that Thoreau and later botanists recorded. Of the species that Thoreau noted in the mid-19th century, a quarter seem to be missing. A further third are now rare, with only a few plants remaining in the area. Some of the most charismatic wildflowers, like many species of orchids and lilies, have disappeared from the area entirely.

Many factors -- increased development, pollution, roads and larger populations of deer -- have affected the abundance and distribution of plants in Concord. But climate change has clearly influenced which plants can be found there today. Interestingly, while collaborating with colleagues from Harvard, we discovered that the plants whose flowering times were most responsive to temperature -- the ones more likely to bloom early in warm weather -- were the very ones most likely to survive the changes in climate. They maintained healthy population sizes or even increased in abundance. In contrast, plant species that were unable to ''track'' changes in temperature in this way tended to decline or disappear, and have been replaced by non-native invasive species like purple loosestrife and garlic mustard, as well as native species from more southerly climes, like sweet pepperbush and silky dogwood, which are better at adapting to warmer temperatures.

This points to something else Thoreau's realometer guards against. Walking through the woods of Concord, it's easy to notice and be alarmed by the more extreme changes in flowering times of species like the highbush blueberry. But the species we should really be concerned about, like columbine, Canada lily, wild cranberry and the small purple fringed orchid -- those with less changeable flowering times -- are the ones we're more likely to miss. For those species, we won't find surprising changes in the dates of their first blooms. But eventually, in many places, we might not find them blooming at all.

Despite their dramatic cumulative effects over the last 160 years, these changes would be largely imperceptible without the biological yardstick Thoreau's records provide. Many others throughout the history of this country have kept diligent records -- their own realometers -- of flowering, bird migrations, butterfly emergence, fish runs and the like. More should be closely examined to lend insight into how changes in climate are affecting the world around us.

As Thoreau wrote, ''The question is not what you look at, but what you see.''

DRAWINGS: HIGHBUSH BLUEBERRY: This common wetland shrub is valued for its edible berries. Its first flowering date in Concord has shifted to early April from mid-May.; RHODORA: A wild native azalea with purple-pink flowers. In Concord, the species is declining in abundance.; MARSH MARIGOLD: This streamside herb produces large yellow flowers above its heart-shaped leaves. Typically an April bloomer, it can flower in March in warm years.; BIRDFOOT VIOLET: This species is rare in Concord and flowers in early May in Sleepy Hollow Cemetery, where Thoreau is buried.; SWEET PEPPERBRUSH: A profusion of sweet smelling flowers in summer is characteristic of this wetland species, found commonly in the Southern United States. It is now increasing in abundance in Concord.; FLOWERING DOGWOOD: This small tree is often planted for its large white flower clusters, which open in the early spring and can sometimes be found in Concord's forests.; GARLIC MUSTARD: A biennial herb with white, four-petaled flowers. It is native to Europe and is now greatly increasing in abundance in Concord.; CANADA LILY: A tall plant with large yellow flowers that often grows in wet meadows. Once common, it is now very rare in Concord.; ROSE POGONIA: A delicate orchid with a single rose-colored flower at the top of a stalk with one leaf. Formerly common in the bogs of Concord and now rare. (DRAWINGS BY BECCA STADTLANDER